1. Field of the Invention
The present invention relates to a manufacturing method for a structured material; and a magnetic recording medium, a magnetic recording and reproducing apparatus, an information processing apparatus, and a manufacturing method therefor. More specifically, the present invention relates to a manufacturing method for a magnetic recording medium using anodically oxidized aluminum. In particular, the present invention relates to a manufacturing method for a patterned medium or a discrete medium in which a magnetic material portion of a recording layer is partitioned into plural portions by partition walls of non-magnetic material.
2. Related Background Art
A recording density of a hard disk used as an external storage device for a computer has been continuously increasing at an annual rate of 100% with the support of progress in an advanced microfabrication technique or signal processing technique, and the like. Recently, a product, which makes recording at an extremely high-density of approximately 40 Gbit/in2 possible, has also been released.
On the other hand, the hard disk has started to be used in full-scale in digital home electrical appliances such as a hard disk recorder and a car navigation system, making use of advantages of the hard disk, such as large capacity, nonvolatility, high reliability, and high-speed access. Thus, a high-density magnetic recording medium, which is smaller in size and larger in capacity, is required.
Currently, a recording system used in the hard disk is a longitudinal recording system for recording magnetizations continuously in a direction parallel with a substrate, which is a so-called in-plane recording system. In the in-plane recording system, reproduction is performed by a magnetic head using a leaked magnetic field from a magnetized transition region provided between magnetized recording sections adjacent to each other. The magnetized recording section is constituted by plural magnetic particles, and these plural magnetic particles are used to record one bit in the magnetized recording section.
However, it is considered that, if a recording density will further increase in future as described above, the recording density will soon reach a physical limit with the current system. This is because, since a recording region for one bit becomes smaller as the recording density becomes higher, it is necessary to reduce a size of the magnetic particles forming the recording region to clarify borders among bits in order to secure sufficient S/N. Moreover, since the leaked magnetic field decreases due to an influence of a demagnetizing field, it is also necessary to reduce a thickness of a magnetic layer. Therefore, it is anticipated that a volume of the magnetic particles is extremely reduced and a region with the recording density exceeding 200 Gbit/in2 falls into a superparamagnetic state in which magnetization directions cannot be maintained due to an influence of thermal energy.
As means for avoiding such a situation and making higher-density magnetic recording possible, a vertical magnetic recording system, which uses a magnetic material having magnetic anisotropy in a vertical direction with respect to a substrate as a recording layer and records magnetizations in the vertical direction, is considered effective.
The vertical magnetic recording system has such a characteristic that, in contrast to the in-plane recording system, the demagnetizing field decreases as the recording density increases. Thus, since a magnetic layer can be kept thick, the vertical magnetic recording system is more advantageous with respect to superparamagnetism than the in-plane recording system. In the vertical magnetic recording system, Co—Cr alloy is generally used as the recording layer. When the Co—Cr alloy is formed on the substrate by the sputtering, Co and Cr grow with compositions thereof separated. A portion with a large quantity of Co component is columnar and becomes a ferromagnetic portion having a hexagonal closed-packed structure (hcp structure) to function as a recording section. A portion with a large quantity of Cr component, which grows so as to surround the columnar recording section, is a non-magnetic portion and also functions to weaken a magnetic interaction between recording sections adjacent to each other.
However, even in the vertical magnetic recording system, since a segregation structure of Co—Cr is used, a size and a shape of the recording portion lack uniformity, and it is extremely difficult to arrange the recording sections regularly. Therefore, borders among bits have an irregular shape in an ultra-high density region, and decrease in S/N is also apprehended.
Thus, for example, a next-generation magnetic recording medium called a patterned medium is attracting attention (Japanese Patent Application Laid-open No. 2000-277330 (page 5, FIG. 3)). The patterned medium is a magnetic recording medium, in which magnetic domains independent from each other are formed by regularly arranging magnetic materials with the same size and shape on a substrate, and one bit is recorded on each magnetic domain. In this case, each magnetic domain that records one bit has an independent shape, and therefore, even if magnetic particles are large, a border with a neighboring bit has a clear and uniform shape. Thus, decrease in S/N is not caused. In other words, the magnetic recording medium is more advantageous with respect to superparamagnetism than a continuous medium of an identical recording density. It can be said that the magnetic recording medium has a structure suitable for ultra-high density recording.
Next, since the present invention uses an anodically oxidized aluminum nanohole formed by anodic oxidation of Al, the anodically oxidized aluminum nanohole will be hereinafter described.
When an Al substrate is subjected to the anodic oxidation in an acid electrolyte of sulfuric acid, oxalic acid, phosphoric acid, or the like, an anodic oxide film, which is a porous anodic oxide film, is formed. This porous film is characterized by having a unique geometrical structure in which extremely fine columnar microholes (nanoholes) with a diameter of several nm to several hundred nm are arranged in parallel with each other at an interval of several tens nm to several hundred nm. The columnar microholes have a high aspect ratio and is excellent in uniformity of a depth and a diameter of a section thereof.
In addition, it is possible to control the structure of the porous film to some degree by changing conditions of the anodic oxidation. For example, it is known that it is possible to control an interval, a depth, and a diameter of the microholes to some degree with an anodic oxidation voltage, an anodic oxidation time, and pore widening treatment, respectively. Here, the pore widening treatment is etching treatment of alumina. Usually, wet etching treatment with phosphoric acid is used as the pore widening treatment.
Moreover, there is also proposed a method of forming starting points for formation of microholes using a stamper in order to improve controllability of a shape, an interval, and a pattern of microholes of a porous film. This is a method of forming pits, which are created by pressing a substrate provided with plural projections on a surface thereof against a surface of an Al substrate, as starting points for formation of microholes and then subjecting the pits to anodic oxidation to create a porous film having microholes showing better controllability of a shape, an interval, and a pattern (Japanese Patent Application Laid-open No. H10-121292 (page 9, FIG. 8)).
It has already been publicly known to apply nanoholes to a magnetic recording medium such as a patterned medium by filling a magnetic material in the above-mentioned anodically oxidized aluminum nanoholes (Japanese Patent Application Laid-open No. H11-224422 (page 7, FIG. 1)).
However, since the nanoholes are usually columnar as described above, inconvenience occurs in the case in which magnetizations of the filled magnetic material face to an in-plane direction of a substrate. In other words, since sections of the nanoholes are circular, it is likely that the filled magnetic material cannot obtain a stable direction in terms of energy according to shape anisotropy in the in-plane direction, and the magnetizations rotate in the plane, or the magnetizations are not arranged in a track direction. This means that a relative positional relation between a direction of magnetizations of the magnetic material filled in the nanoholes and a magnetic head that detects the direction of magnetizations changes for each nanohole, and the magnetic recording medium cannot perform recording and reproduction accurately.
In addition, there is also proposed to, by forming a magnetic domain in a rectangular or elliptical shape, give shape anisotropy to magnetic particles to improve a coercive force and to arrange magnetizations in a track direction (Japanese Patent Application Laid-open No. H6-028093 (page 3, FIG. 1)).
However, in this method, a drawing process by micro beams such as electron beam lithography is required for regularly arranging magnetic domains. Although accuracy of finishing of the electron beam lithography is extremely high, a throughput thereof is poor because it is not collective lithography like photolithography. Thus, it is extremely difficult to apply treatment to a large area. In other words, it can be said that a method of patterning magnetic domains one by one with the electron beam lithography is unrealistic in terms of productivity in the current technique.